Rotary impact power tools are used to tighten or loosen fastening devices such as bolts, nuts, screws, etc. Rotary impact power tools have been developed that use a pneumatic or electric motor to drive a hammer which rotationally impacts an anvil. These anvils typically have a tang portion with a square cross section and are coupled with an output such as a drive socket. The tang portion has a transverse hole on one of the faces to house a spring-loaded detent pin. The detent pin releasably engages a corresponding recess in the drive socket.
Prior art anvils used in impact drivers are subject to fatigue failures. Fatigue is a phenomenon that leads to fracture in a load-bearing member under repeated or fluctuating stresses, even though those stresses may be substantially less than the tensile strength of the member. Fatigue fractures generally start at a point of geometric discontinuity or stress concentration and grow incrementally until a critical size is reached. It has been found that a stress concentration is created at the transverse hole on the face of the anvil tang in prior art anvil designs. This stress concentration at the transverse hole severely weakens the anvil tang, increasing its risk of fatigue failure. Further, when the anvil tang is subject to a fatigue failure, the failure can occur in a catastrophic manner. This potentially results in propelling the socket and broken portion of the anvil at high speed, which may injure an operator or bystander.
For the foregoing reasons, there is a need for an anvil for an impact driver that reduces the stress concentration and fatigue failure at the tang.